Let's address a few issues.
Amazon has ever-changing standards. Why? Because they have erotica authors who are using ever changing tactics to get their books out of the adult dungeons and seen by as many people as possible. Because they have customers reacting to these books, and not reacting in a good way. Because KDP makes them a shitload of money, and ensuring that the public perceives that store in a good light is in their economic self-interest.
Let's go beyond that. Do you remember all the talk about "rape culture" that's happening and why? Well, as far as Amazon can tell, there it is. With the female community, the biggest buyer of books, growing concerned if not militant about rape culture, do you think that Amazon can stand by and let these same works crop up in their romance searches? Do you think that they are going to sacrifice millions of dollars in profit to protect dark erotica's thousands of dollars in profit?
Amazon is a business. It has the right to decide what to stock in its store. It has a need to protect its reputation. In the ever-changing marketplace, it will address the problems that arise. That's it. That's as complicated as it can get.
As an author, I have no right to be published through the KDP. I am fortunate that Amazon has created rules that allow me to do so. One day, this will certainly change because the business environment will change. I personally don't expect the golden age of KDP to last another decade. Nothing is infinite. For now, we have the market that we have. Tomorrow, we have another market.